Petroleum explorationists often find it desirable to determine detailed information about an existing well. The standard technique for obtaining information such as resistivity, porosity, and density along the entire extent of the well is to lower an appropriate "logging" tool with a control cable or "wireline" to the bottom of the well and then slowly pull the tool up the full extent of the well while making accurate measurements of the desired parameters as well as recording the progressive tool location. This "well logging" technique has been developed to a very precise art in the petroleum industry.
A critical aspect of successful well logging is the accurate measurement of the location of the downhole logging tool. The accuracy must be especially precise for certain tools such as a borehole gravity meter. The precise measurement of tool location in deep wells can be, and usually is, a delicate operation. The most significant problem encountered is the tendency for the wireline to elastically deform and stretch under its own weight and the weight of the downhole tool. It is therefore necessary to carefully control the rate of the initial acceleration and eventual steady state velocity of the wireline as it is drawn up the length of the well in order to prevent the tool from "bobbing" in the well (which could be caused by uncontrolled fluctuations in the wireline tension). Suitable techniques, which are in current use throughout the petroleum industry, have been developed to overcome the above-mentioned difficulties during the logging of land-based wells.
A new dimension is added to the wireline stabilization problem when the logging operation is conducted on an offshore well from a floating vessel. In particular, the waves and swells of the sea water can cause the logging vessel to pitch, roll, bob, and sway relative to the wellhead at the ocean floor. These undesirable motions can occur even if the offshore logging is being done from a platform connected to or raised from the ocean floor. If such motion is not in some way eliminated from the wireline near the top of the well, the downhole tool may be caused to bob in an undefined manner such that a precise measurement of the location of the tool will be impossible. Furthermore, even if the location of a downhole tool is measured to the desired degree of accuracy, the tool itself may be of the type that will produce a meaningful reading of a particular well parameter only if it is stationary in the well. With such a tool no motion of the wireline in the well is tolerable.
Compensators, similar to the Rucker drillstring compensator, are available which can be used to substantially decrease fluctuations in wireline tension experienced while logging from an offshore rig. Unfortunately, these compensators are inadequate to sufficiently insulate the wireline from unacceptable motion under even moderately severe weather conditions. The applicants are not aware of any method or apparatus, other than the instant invention, which is capable of providing adequate wireline stabilization and logging tool location for offshore well logging operations under conditions of moderate to heavy seas.